The invention relates to a magnetic recording medium and a magnetic recording apparatus, to which a perpendicular magnetic recording technology is applied, and in particular, to a magnetic recording medium having areal recording density not less than 23.2 gigabits per one square centimeter, and a magnetic recording apparatus using the same.
With a longitudinal magnetic recording system for use in the present magnetic disk drive, there is the need for decreasing a demagnetization field in recording bits by reducing a product of remanent magnetization of a magnetic film, which is a recording medium, and the thickness of the magnetic film in order to enhance resolution. In addition, because there is the need for reducing a crystal grain size of the magnetic film in order to lower medium noise, it becomes essential to reduce the volume of magnetic grains in the magnetic film. In this connection, if the volume of the magnetic grains is reduced, magnetization of the magnetic grains fluctuates due to the effect of ambient heat, resulting in loss of recorded information, so that a problem of so-called thermal demagnetization becomes pronounced. In the past, thermal demagnetization has been suppressed by enhancing magnetic anisotropy of the magnetic film; however, it has become hard to further enhance the magnetic anisotropy of the magnetic film because of limitations to the strength of a magnetic field emanating from a recording head. Subsequently, it has been hard to attain recording density in excess of 23.2 gigabits per one square centimeter with the use of the longitudinal magnetic recording system.
Attention is being focused on a perpendicular magnetic recording system as a method for solving these problems. The perpendicular magnetic recording system is a system for forming recorded bits such that magnetization of a recording medium is effected so as to be normal to a medium plane and magnetization in the recorded bits, adjacent to each other, becomes mutually anti-parallel. With the perpendicular magnetic recording system, a demagnetization field in a magnetization transition region is small, so that a steep magnetization transition region is formed as compared with the case of the longitudinal magnetic recording system, thereby stabilizing magnetization at a high density. Accordingly, a film thickness can be increased to obtain the same resolution as compared with the case of the longitudinal magnetic recording system, thereby enabling the thermal demagnetization to be suppressed. Further, by combining a perpendicular magnetic recording medium having a perpendicular magnetic recording layer and a soft-magnetic underlayer with a single pole type recording head, a high recording field can be obtained, and material high in magnetic anisotropy can be selected for the perpendicular magnetic recording layer, enabling the thermal demagnetization to be further suppressed.
For a magnetic recording layer (magnetic layer) of the perpendicular magnetic recording medium, use of a Co—Cr—Pt base alloy film being used in the longitudinal magnetic recording medium as well, or a superlattice multilayer with Co and Pt, alternately stacked in a multitude of layers, and so forth is under study. In addition, a proposal has been made on use of a granular medium having a structure in which individual magnetic grains are magnetically isolated, and the magnetic grains columnar in shape are surrounded by a non-magnetic compound, such as an oxide, a nitride, and so forth, in order to lower medium noise. For example, in JP-A No. 342908/2002, there has been disclosed a medium obtained by adding an Si oxide containing Si in a range of 8 to 16 at. % in terms of Si atomic weight to a Co—Cr—Pt alloy.
With the perpendicular magnetic recording system as well, it is necessary to render the crystal grain size small and uniform in order to lower medium noise. Further, if the recording layer is excessively large in thickness, a recording field emanating from a head becomes small. For this reason, in order to attain a still higher recording density, the magnetic grains need be reduced in volume, thereby causing the thermal demagnetization to be pronounced. As with the longitudinal magnetic recording system, a magnetic field emanating from the head is limited, so that there are limitations to enhancement in magnetic anisotropy of the recording layer.
In order to solve this problem, a medium with an axis of easy magnetization of a recording layer, tilted in relation to a recording field, has been proposed as a means for lowering a recording field required for magnetization reversal of the recording layer. For Example, in IEEE Transactions on Magnetics, Vol. 38, No. 6, November 2002, pp. 3675-3683, “Magnetic Recording Configuration for Densities beyond 1 Tb/in2”, there is described a perpendicular magnetic recording medium with an axis of easy magnetization of a recording layer, tilted 45 degrees from a direction normal to a medium plane. It is described that, with the medium, as a result of the axis of the easy magnetization being tilted in relation to a recording field, a magnetic field required for magnetization reversal is considerably reduced, so that magnetic anisotropy energy of the recording layer can be considerably enhanced, thereby enabling a high recording density to be reconciled with high thermal stability.